Interferometer-polarimeter

ABSTRACT

A system for measuring the intensity and state of polarization of a radiation field as well as obtaining the spectral variations of these quantities with a wide range of spectral resolution, i.e., from low to extremely high resolution values, is disclosed. The system generally includes any standard or conventional twobeam interferometer which is modified by the inclusion of a polarizer in each of the beams and an analyzer positioned in front of a sensor or recording device. More specifically, the system employs a beam splitter which serves to divide light from a selected light source into a pair of individual light beams. Each of the light beams is directed through a polarizer. The polarizers are positioned to have preselected planes of polarization with respect to each other and with respect to the plane of polarization of the analyzer. The polarized light beams are applied to a variable optical retarder which serves to selectively modify the relative optical path lengths of the light beams. An optical mixer may be employed to recombine the two light beams. The recombined light beams are projected through an analyzer, such as a linear polarizer, to a sensor or recording device.

United States Patent Low et al.

[54] INTERFEROMETER-POLARIMETER [72] Inventors: George M. Low, Deputy Administrator of the National Aeronautics and Space Administration, with respect to an invention of; Alain L. Fymat, San Marino, Calif.; Krishna D. Abhyankar, Hyderabad, India 22 Filed: Nov. 13,1970

[21] App]. No.: 89,211

[52] US. Cl ..356/106, 356/114 [51] Int. Cl. ..G0lb 9/02, GOln 21/40 [58] Field of Search ..356/l06, 113

[56] References Cited UNITED STATES PATENTS 3,601,490 8/1971 Erickson ..356/ l 06 3,463,924 8/1969 Culshaw et a1 ..356/106 X Primary Examiner-Ronald L. Wibert Assistant Examiner-Conrad Clark AttorneyMonte F. Mott, Paul F. McCaul and John R. Manning Oct. 24, 1972 [57] ABSTRACT A system for measuring the intensity and state of polarization of a radiation field as well as obtaining the spectral variations of these quantities with a wide range of spectral resolution, i.e., from low to extremely high resolution values, is disclosed. The system generally includes any standard or conventional twobeam interferometer which is modified by the inclusion of a polarizer in each of the beams and an analyzer positioned in front of a sensor or recording device. More specifically, the system employs a beam splitter which serves to divide light from a selected light source into a pair of individual light beams. Each of the light beams is'directed through a polarizer. The polasizers are positioned to have preselected planes of polarization with respect to each other and with respect to the plane of polarization of the analyzer. The polarized light beams are applied to a variable optical retarder which serves to selectively modify the relative optical path lengths of the light beams. An optical mixer may be employed to recombine the two light beams. The recombined light beams are projected through an analyzer, such as a linear polarizer, to a sensor or recording device.

7 Claims, 3 Drawing Figures 20 /i f 44/152 A 50mm 44/ (TWA/6 fix/776a WW Paten ted Get. 24, 1972 2 Sheets-Sheec'l Patented Oct. 24, 1972 Arron/5,6.

1 INTERFEROMETER-POLARIMETER ORIGIN OF THE INVENTION BACKGROUND OF THE INVENTION I. Field of the Invention v This invention generally relates to devices for performing an analysis of optical radiation. More particularly, the present invention concerns an optical arrangement which may be employed to measure both the intensity and state of polarization of optical radiation as well as to perform a spectrum analysis of these quantities with any desired spectral resolution. 2. Description of the Prior Art It is well known that scattering influences, in a fundamental way, the nature of the radiation field which emerges from a planetary atmosphere. The effects of this scattering depends on both the relative importance or amount of scattering and absorption in attenuating an incident light beam and on the phase matrix for the scattering particles. Conventionally, the phase matrix of a scattering particle defines how that particle will directionally redistribute optical radiation incident on the particle. Since these two characteristics vary with frequency, the spectral variationof the observed radiation, particularly the line profiles, can give considerable information about the atmospheric structure. One relevant factor in such cons derations is the variation of polarization across the line; even a simple measurement of the difference between the integrated polarization within the line and the polarization of the surrounding continuum is important in this respect.

In addition to the study of planetary spectra, determining concurrently both the intensity and state of polarization of light has direct applications to other astronomical problems such as measuring the polarization of emission lines in planetary nebulae, in the airglow spectrum, and in chemical analyses of constituents.

Accordingly, there is a continuing need in the field of optical research for experimental techniques and apparatus that are useful in measuring both the intensity and state of polarization of light with any desired degree of spectral resolution, from low to extremely high. This is particularly true when it comes to quantitatively determining the intensity and state of polarization of a light beam by obtaining the relevant Stokes parameters of light which define intensity (I), degree of polarization (Q), orientation of the plane of polarization (U), and ellipticity of the polarization ellipse (V).

Conventionally, the Stokes parameters canbe obtained by making four suitably chosenmeasurements. For example, total intensity (I) may be obtained by using and 90 polarizers in conjunction with an analyzer. Alternatively, it may also be obtained by Fourier transform spectroscopy using an ordinary unmodified Michelson interferometer. Measuring degree of polarization (0) also. requires the use of 0 and 90 polarizers in conjunction with an analyzer. The remain- 2 ing two parameters U and V using a 45 polarizer and a such as a quarter wave plate,

can be obtained by phase shifting device, in conjunction with an analyzer.

Typically, all four measurements are made by employing a fixed retarder, three polarizers, at 0, 45 and 90, and an analyzer is series optical relationship. However, except for ordinary Fourier transform spectrometry which considers only the total intensity (I), these conventional techniques do not provide the degree of resolution necessary for analysis of the spectra of, for example, relatively unknown gaseous, liquid, or solid media, or media containing elements in some or all of these phases.

OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an optical arrangement that enables both the intensity and state of polarization of optical radiation to be measured with any desired degree of resolution.

It is another object of the present invention to provide an optical arrangement that enables the spectral distributions of both state of polarization as well as energy to be measured concurrently.

It is a further object of the present invention to provide an optical system for obtaining information about the composition, structure and optical properties of an absorbing, scattering or emitting medium.

It is a yet further object of the present invention to provide an interferometer-polarirneter that may be easily employed to obtain from low to extremely high resolution data for very weak emitting sources.

Briefly described, the present invention involves a technique and apparatus that is useful for Fourier transform spectroscopy as well as for determining the state of polarization of optical radiation and its spectral variation to thereby provide the synergistic advantages of having an interferometer as well as a polarimeter. For example, the combined capability of the invention provides the advantage of efficient utilization of light allowing very weak sources to be studied from both standpoints. Additionally, the high degree of resolution allows planetary media and emitting sources, heretofore unable to be studied, to be analyzed.

More particularly, the subject interferometer arrangement includes a standard two-beam interferometer which plays the role of a continuous phase retarder and is equipped with matched, and particularly oriented, polarizers in each beam. An analyzer is situated at the recombined focus point. of the interferometer. A sensing or recording device is positioned to record interferograms. The interferometer may include a beam splitter for providing the pair of light beams, and an optical retarder for allowing the relative optical path lengths of the two light beams to be varied. A

beam recombiner in the form of a functionally reversed beam splitter may be used to recombine the two light beams. This recombination may also be performed directly as in a Youngs slit device.

The intensity and state of polarization of optical radiationare measured by obtaining three interferograms. For each interferogram, the polarizers are positioned to have selected planes of polarization with respect to each other and with respect to the analyzer.

The resulting data is simply employed in the calculation of the Stokes parameters.

The features that characterize the novelty of the present invention are set forth with particularity in the appended claims. Both the organization and manner of operation of the invention, as well as other objects and the attendant advantages thereof, may be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings wherein like reference symbols designate like parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram illustrating a preferred embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a Michelson interferometer that has been modified in accordance with the present invention.

FIG. 3 is a graphic diagram illustrating a number of different interferograms along with corresponding energy spectrums or Fourier transforms.

DESCRIPTION OF THE PREFERRED EMBODIMENT is in terms of the time averages (denoted by angular brackets) of products of the complex field components E, E*,- J where i= x,y andj x,y and where the asterisk( designates the complex conjugate of the term E It has been established that these complex field components also define a coherency matrix where E denotes the Hermitian conjugate of and the cross represents the Kronecker product. The elements of the coherency matrix Q are related to the Stokes parameters of the radiation field as follows:

1 =1 J,,,, Trace I It may be noted that J and J are real quantities whereas! and J are complex conjugates of each other. In that 1,, may be conveniently expressed in terms of real quantities, i.e., I, exp (iB all of the Stokes parameters can be considered as real quantities and have the dimensions of intensity.

The effect of any optical device on an electric field can be represented by a 2 X2 Jones matrix I such that E' =15? 3 The coherenc matrix, 1', of the resultant field can be obtained from E by using Eq. 1 The emergent intensity given by the trace Tr I' of the matrix ,1 is a function of the Stokes parameters of the incident field. For example, if light is passed through a compensator, which retards the phase of the y-component with respect to that of the x-component by an angle 6, and then passed through a linear polarizer making an angle 0 with the positive x-direction, the emergent intensity will be given by the interference equation:

I(0,e) J cos 0+ J sin 0+ 2 [1 cos 0 sin 0 cos(,B e). 4 From this equation (4) it is readily apparent that four independent measurements of I(0,e) for various combinations of 0 and s will be sufficient to determine the four Stokes parameters (I,Q,U and V).

Referring now to FIG. 1 of the drawings, the present invention includes a two-beam interferometer comprising a beam splitter 10, a variable retarder l2, and a beam recombiner 14. An appropriate sensing or recording device 16 may be situated at the focal point 18 to sense or record emergent light. An analyzer 20, which may be a linear polarizer, is situated intermediate the beam recombiner l4 and the recorder 16. A pair of matched polarizers 22 and 24 are respectively situated in the paths of the two light beams produced by the beam splitter 10.

The beam splitter 10, as well as the beam recombiner 14 (when a functionally reversed beam splitter is used therefor), must be suitably chosen to accommodate the region of the electromagnetic spectrum in which a particular experimentis being conducted. For example, beam splitters have been developed to accommodate different portions of the infrared region. In the intermediate infrared region (1.5 p. to 20 t), where Fourier spectroscopy possibly has its greatest advantages, a typical beam splitter may consist, for example, of 'a thin film coating, such as germanium, which is structurally sandwiched between a transparent substrate and compensating plate. Usually the substrate and compensator are made of a low refractive index (n) material such as sodium chloride (n 1.5

The variable retarder 12 may simply include any well known combination of elements that serve to vary the optical path length of one of the two light beams produced by the beam splitter 10 such that the difference between the respective optical path lengths can be varied. In a Michelson interferometer, as shown in FIG. 2, the var'iable retarder 12 comprises a pair of mirrors, of which one is fixed and the other is movable along the path through which incident light is projected.

The polarizers 22 and 24 serve to suitably polarize the light beams projected therethrough. Although the polarizers 22 and 24 may be arbitrarily chosen, it is preferable for the sake of simplicity that both polarizers either be linear or circular. Generally considered, circular polarizers are of limited interest because it is, as a practical matter, difficult to construct circular polarizers which are perfect over a large range of wavenumbers.

Each of the polarizers 22 and 24 are appropriately mounted to allow the respective planes of polarization to be varied with respect to each other. This simply requires that the polarizers each be adapted to be 5 6 rotated about the path along which a light beam is proin the form of a linear polarizer. Letting Z represent ected. the Jones matrix for the analyzer 20, the amplitude of The analyzer 20 may also a linear polarizer, the the combined light beams emerging from the analyzer plane of polarization of which is set along a predeter- 20 will be E(Z 2;, (E M, Z, R M Z 8 mined axis as is hereinafter described. 5 which gives the coherency matrix ,1 (6) according to The present invention may incorporate a Michelson Eq. (1). The contribution to intensity for a fixed path interferometer, a Young interferometer, or any other difference r,from radiation in the wavenumber interval two-beam interferometer. FIG. 2 illustrates the fashion dowill be IT(U') do= Tr ,[(8)d-r. The total intensity is in which a Michelson interferometer may be used. A given by pair of mirrors 26 and 28, of which the mirror 26 is fixed and the mirror 28 is movable, are employed to 1(1) f I (a) dd produce the necessary variations of the relativepath (6) lengths of the two light beams produced by the beam splitter 10. The polarizers 22 and 24 are respectively l wh rein the variable part of I (1') is known a th i interposed between the beam splitter 10 and the mirferogram. Although both the variable and constant mm 26 n parts of l (1) contain information about the Stokes Refemng Once agam to 35 from parameters of the original radiation, only the interferothe source 11 may be expressed as where 0' ram canbe usedfrthi t designates the wavenumber of the radiation. The two 8 0 S purpose m he methgd of b d d b th b Ht 10 b Fourier spectroscopy.

eams pro uce y e earn Spl er may e e x- Refemn now to HG. 3 l t rf pressed as complex amplitude vectors S E and g, E g exemp ary m e erograms h S d S th J t fig and corresponding Fourier transforms are illustrated. l cfiifizil'fietiifi hZTffifififZZ? $333.1? in; As by warm imerfemgmm p monochromatic light would appear as a pure sine wave the fractional transmission, or reflection, and the polarization introduced into the beams. (For an ideal when mtenslty I (7) plotted as functlon of path Michelson interferometer s1 =2 V71 where l is ference 'r. The corresponding energy spectrum appears the unit matrix). The light beams 1 and 2 are then as a Spike as l y P q A The interferopassed through the polarizers 22 and 24, respectively. gram for cluas"monochromatlc hght would appear as a Letting Z and Z represent the Jones matrices of the bluned wave as shown by waveform The two polarizers 22 and 24, the amplitudes of the light {espondmg energy Spectrum quasljmonochmmatlc beams emerging front the polarizers may be expressed generally appears as a senes of sink s shown by a Z1 S1 i d Q 2 E Waveform B. As shown by Waveform C, the interfero- The interferomete r introduces a relative retardation l' forrpolychmmatic light pp as an p between the beams 1 and 2 which retardation may be y attenuated sinuscidal Wave when I iS P e represented by th J s mat i g, d g If th as a function of path difference, r. The corresponding relative retardation in phase is 8=21rm-, where 7 energy spectrumis Show" by waveform equals path difference, one way of expressing B In order to obtain the Stokes parameters Of the ina d R would be cident radiation from the interferograms, it is important to first examine how theyenter into the observed 31 L 2: 7 5 interferograms.

After retardation, the two light beams l and 2 have Letting P,=1 l Z (j=1, 2), P Z writing the amplitudes B, Z, 5, E and B Z 5 E The light P; P- beams are then recombined, either directly (as in ?i(') i; 29 (i=1, 2, 3)

and making use of Eq. (5), we get the emergent amplitude vector 1 P P P P P P E E P u u 21 22) e x) P15 P14 its/2+ P23 P24 16/2 7 Et It can be written as 6 6 6 6 (a; cos 13 sin E (a; cos -15 S111 E Young's experiment) or after a second passage through where the a s and B s are functions of a through the the splitter (as in Michelsons interferometer), by the P s. Then computing {(8) and taking its trace, we obbeam recombiner 14. The effect of the recombiner 14 0 tain a can be represented by the Jones matrices M, and M q i for the two beams, respectively. Hence, the combined 1( 1 2 3( Sin 9 emergent amplitude will be (M B Z, S M B Z 5 Where (after using q ,01'(B1M1Z1 Sn E2 M2 225.10 to Eq- M =pr( K +qr( Qwl w) i( n the radio region, it is possible to deal directly with V(rr),

this combined amplitude in the form of an output volt- (i l, 2, 3), 10

age of the mixer element of the circuit. However, in the and p,, q,, r,, s, are known real functions of the p s optical region, it is necessary to employ an analyzer 20 through the as and ES. The observed intensity will be d(o) 1 where d(0') is a factor representing the instrumental sensitivity.

Introducing the auxiliary quantities p(o') and #:(0') by the equations a (o') p(a) cosu,b(a-) in the cosine term at the right hand side of this equation, and to weight the constant and variable parts of the intensity with different functions of 0-.

In the case of quasi-monochromatic light, for example, the spectrum consists of an extremely narrow band centered at wavenumber 0- Hence Eq. 12) itself gives the observed intensity with 0 0 and 6 27ra' 'r. It is a sinusoidal function of r from which the quantities a, (d can be derived in a straightforward manner by using Eq. (11). Then Eq. (10) provides three linearrelations between the four unknown Stokes parameters. Hence, one intensity curve alone is not sufficient for deriving all of these parameters. But, if one more intensity curve with an independent setting of the polarizers 22 and 24, and the analyzer 20, is obtained, three additional relations will be obtained. Combining the two data, we have six equations which are more than sufficient to determine the four Stokes parameters.

Considering the use of Fourier spectroscopy for polychromatic light, total intensity is obtained by integrating l,(o') over all wavenumbers. Thus, substituting Eq. (l2) in Eq. (6), we get and the int'erferogram is given by This expression shows that, owing to the use of the polarizers 22 and 24, the interferogram obtained with the present invention is not a symmetrical function of 1'. Hence, the spectral distribution of the incident radiation is not the Fourier cosine transform of the interferogram. We must therefore resort to the full (exponential) transform for deriving p(o and 111(0'), which in turn would give the four Stokes parameters of the incident radiation.

If for negative wavenumbers we assume that we have then Eq. (14) becomes:

Eq. (16) is true only for an idea] interferometer. In practice, however, one has to take into account the following imperfections of the system: i

i. The finite extent (7,, T of the interferogram; this is indicated by a positive transfer function T('r) in .front of the integral on the right hand side of Eq. (14-). This function is made to vanish outside the interval (1,, r

ii. The zero point of r may vary with 0' which introduces a further asymmetry in the interferogram; this is taken into account by introducing a phase term i (0)} in the argument of the cosine factor. Amplitude terms which depend on 0' only can be'included in d(0').

iii. On account of (ii), the values of 1', and 1- may vary with 0-, although their difference (7 r may remain constant, thus causing changes in the geometrical parameters of the interferometer. This interaction gives rise to another positive amplitude factor B( rr) and an additional phase term 'y(o-r). Hence Eq. 14) becomes 4 =d r). 0"), then, the interferogram could be expressed by can be replaced by 8* (0- 7), where 0' is the mean wavenumber of the passband. Then, Eq. (19) becomes 2 Var [I(1-)] WW=1 a exp From Eq. (21) we see that 2Var[I(r)]/T('r) S*(0' r) and 0( X p 1'( 1 form a Fourier pair. Hence the full (exponential) transform of the interferogram yields the latter function. In Eq. (21 the function T(r) represents an amplitude factor, for the interferogram is real. It is the Fourier transform of the scanning function. If the latter is apodized, then Y(r) is a suitably tapered function which vanishes at the ends of the path difference interval (r r in a smooth way. The phase term (cr) represents an important source of error for it contributes, along with 41(0), to the destruction of the symmetry of the interferogram. The term S( rr), known as the source function, includes both amplitude and phase effects. Now, since T(1-) d(o'), and S*0' 1') are known functions, one can derive p(0) and IIJ(O')+ (o')]. In order to determine the phase error (o'), we must have one interferogram for which M0") is known. Here it is assumed that the interferometer is sufficiently stable during the time of one set of measurements so that 41(0) is identical for all interferograms. Of particular interest are the caseswhere 41(0) 0 which are later discussed.'For them 42(0) can be obtained by well known methods. However, this determination of 42(0) is arbitrary to within an additive term 21mm which depends on the choice of the origin of the interferogram. If the same origin is taken for allinterferograms, this additional term will be identical for all of them.

Now, any interferogram will give us the two functions p(0') and 41(0) 41(6) 211-07; and since (12(0) 211-94 1- is known, we can obtain 11(0), the phase term introduced by the polarizers. The quantities p(o) and M0), in turn, yield a (0') and a (o-) according to Eq. (11). Then, from Eq. (10), we see that we have two relations in the four unknown Stokes parameters. Hence, in addition to the interferogram yielding (o'), we must have two other independent interferograms for deriving all the four Stokes parameters.

The Jones representation for a perfect linear polarizer making an angle 6 with the x-axis is given by the real matrix cos 0 sin 0 sin 0 (22) where f=f;, (i l, 2, 3), are the transmission factors. Then, putting 0 6, and carrying out the computations represented by Eqs. (8) to (10), we find that s (9) w: (9) q (74 r w) 0 in Eq. (10) Hence, a (.0) is a function of V(0) alone and a contains [(0), ()(0) and U(0) only. In other words, V(0) gets separated from the other three Stokes parameters. Consequently, no two settings of P and P are completely independent of each other. Any one setting gives an interferogram which by the Fourier spectroscopic method yields V(6) and a linear relation between the remaining three Stokes parameters. Hence, three interferograms would be needed to get complete information about the intensity and state of polarization of the incident light.

TABLE I., hereinbelow, is provided as a summary of the equations that are pertinent to three different combinations of settings for the analyzer 20 and the polarizers 22 and 24 which could be used to obtain the three interferograms required to calculate the Stokes parameters. In TABLE 1., the polarizers 22 and 24, in

combination with the variable retarder 12 and the beam splitter 10, in both its direct and reversed functions, are designated by l and P respectively, as earlier defined in connection with Eq. (7). The analyzer 20 is designated by P The terms 1:, y and s refer to planes of polarization equal to 0, 90 and 45, respectively.

. TABLE I.Interfcrometric Arrangements with Linear Polarizers 2c--- Amplitude 'E P, =P,(E,f,+,f,)-

3---- Cohereney matrix {(6) 4---- Intensity= Tr {(6) in terms of the elements of the original coherency matrix J.

5 Intensity in terms of the Stokes parameters of the incident radiation.

f w+1==+ fm cos G i m AE. 2b fzE a y, s

fl x fa v (f1 +fc f1fc coo em 1 1 From the foregoing discussion, it is now apparent that a two-beam interferometer, equipped to have a matching polarizer in each light beam and an analyzer in the form of a linear polarizer will produce interfero grams from which the Stokes parameters may be obtained by the simple expedient of obtaining three different interferograms as explained above. The high degree of resolution capable of being obtained with the present invention now makes possible the analysis of spectra of media heretofore impossible.

What is claimed is:

1. Apparatus for producing interferograms from which the four Stokes parameters defining both intensity and the state of polarization of incident radiation, and special variations thereof, can be concurrently determined, said apparatus comprising: a two-beam interferometer including: splitter means for dividing incident radiation into two beams,

retarder means for controllably producing phase differences between said two beams, and recombiner means for combining said two beams into a single recombined beam of radiation; first and second polarizers, each positioned in a respective one of said two beams and interposed between said splitter means and said retarder means, said first and second polarizers being adapted to have the polarization angles thereof selectively modified; and a third polarizer positioned in said recombined beam of radiation, said first, second and third polarizers being successively positioned to form preselected combinations of polarization angles, an interferogram being obtained for each successive combination to permit concurrent determination of said Stokes parameters.

2. The apparatus defined by claim 1 wherein said first, second and third polarizers are linear polarizers, said polarizers being adapted to be adjusted to polarize at any angle light projected therethrough.

3. The apparatus defined by claim 1 wherein said first and second polarizers are circular polarizers and said third polarizer is a linear polarizer, said first and second polarizer together forming selected combinations of right and left circular polarizers.

4. The apparatus defined by claim 1 wherein said interferometer is a Michelson interferometer.

5. The apparatus defined by claim 1 wherein said retarder means comprises first and second mirrors positioned in a respective one of said two beams for reflecting incident radiation, one of said first and second mir rors being stationary and the other of said mirrors being adapted to be moved controllably along the path of incident radiation, the relative phase of said two beams beingrtfiereby controlled.

6 e apparatus defined by claim 5 wherein said first, second and third polarizers are linear polarizers, said polarizers being adapted to be adjusted to polarize at any angle light projected therethrough, said interferometer being a Michelson interferometer.

7. The apparatus defined by Claim 5 wherein said first and second polarizers are circular polarizers and said third polarizer is a linear polarizer, said first and second polarizers being changeable to together form selected combinations of right and left circular polarizers, said interferometer being a Michelson interferometer.

lnuun 

1. Apparatus for producing interferograms from which the four Stokes parameters defining both intensity and the state of polarization of incident radiation, and special variations thereof, can be concurrently determined, said apparatus comprising: a two-beam interferometer including: splitter means for dividing incident radiation into two beams, retarder means for controllably producing phase differences between said two beams, and recombiner means for combining said two beams into a single recombined beam of radiation; first and second polarizers, each positioned in a respective one of said two beams and interposed between said splitter means and said retarder means, said first and second polarizers being adapted to have the polarization angles thereof selectively modified; and a third polarizer positioned in said recombined beam of radiation, said first, second and third polarizers being successively positioned to form preselected combinations of polarization angles, an interferogram being obtained for each successive combination to permit concurrent determination of said Stokes parameters.
 2. The apparatus defined by claim 1 wherein said first, second and third polarizers are linear polarizers, said polarizers being adapted to be adjusted to polarize at any angle light projected therethrough.
 3. The apparatus defined by claim 1 wherein said first and second polarizers are circular polarizers and said third polarizer Is a linear polarizer, said first and second polarizer together forming selected combinations of right and left circular polarizers.
 4. The apparatus defined by claim 1 wherein said interferometer is a Michelson interferometer.
 5. The apparatus defined by claim 1 wherein said retarder means comprises first and second mirrors positioned in a respective one of said two beams for reflecting incident radiation, one of said first and second mirrors being stationary and the other of said mirrors being adapted to be moved controllably along the path of incident radiation, the relative phase of said two beams being thereby controlled.
 6. The apparatus defined by claim 5 wherein said first, second and third polarizers are linear polarizers, said polarizers being adapted to be adjusted to polarize at any angle light projected therethrough, said interferometer being a Michelson interferometer.
 7. The apparatus defined by Claim 5 wherein said first and second polarizers are circular polarizers and said third polarizer is a linear polarizer, said first and second polarizers being changeable to together form selected combinations of right and left circular polarizers, said interferometer being a Michelson interferometer. 